This invention relates to a viscoelastic damper which is suitable for controlling the vibrations of structural members and in particular to such a viscoelastic damper which offers a relatively small resistance to a low speed motion and a high damping effect to a high-speed vibrational motion.
In various facilities such as nuclear and other power stations, chemical plants, building structures, etc., dampers of various forms are used for the purpose of controlling the vibrational motions of structural members, pipes and tanks which may arise from seismic activities, wind, the motion of fluid contents in pipes and tanks and so on. Normally, such a damper must be be able to allow low-speed motions of the structure which typically arise from thermal deformation of the structure with as little resistance as possible.
Conventionally, such a damper typically consisted of a viscous damper which produces a substantially linear damping effect or a mechanical snubber which makes use of an inertia element. The viscous damper has the advantage of low cost but produces a relatively great resistance to low speed motions and may cause unfavorable reaction forces in the structure to which it is applied. Furthermore, because flow of viscous fluid must be restricted by an orifice or the like in the viscous damper, the pressure of the liquid is substantial and the leakage of the viscous fluid may become a problem. The mechanical snubber has the advantage of presenting very little resistance to a low-speed motion but is very complicated in structure and tends to be expensive.
Japanese Patent Laying-Open Publications Nos. 60-65930 and 60-65931 disclose viscoelastic dampers which make use of dilatant liquid and display a non-linear damping effect. The dilatant liquid presents a very fluid property when the speed of its deformation is small but presents a solid property or greater resistance when the speed of its deformation is great. Therefore, when such liquid is used in a damper for creating a damping effect, one can obtain a damper of a favorable property for the above mentioned applications because the resistance of the damper is great against vibrations and other high speed motions and is small against slow speed motions such as those resulting from thermal deformation of structural members. Furthermore, unlike a conventional viscous damper in which the motion of liquid is restricted by a small orifice or a gap, there is relatively very little pressure build-up in the damper. Therefore, according to such a viscoelastic damper there is less chance of liquid leakage and the sealing structure is simpler because it is required only to deal with liquid of relatively low pressure.